# Search for CP violation in the decays D+--> K(S)pi+ and D+-->K(S)K+.

**ABSTRACT** A high-statistics sample of photoproduced charm from the FOCUS experiment has been used to search for direct CP violation in the decay rates for D+-->K(S)pi+ and D+-->K(S)K+. We have measured the following asymmetry parameters relative to D+-->K-pi+pi+: A(CP)(K(S)pi+) = (-1.6+/-1.5+/-0.9)%, A(CP)(K(S)K+) = (+6.9+/-6.0+/-1.5)%, and A(CP)(K(S)K+) = (+7.1+/-6.1+/-1.2)% relative to D+-->K(S)pi+. We have also measured the relative branching ratios and found Gamma(D+-->K(0)pi+)/Gamma(D+-->K-pi+pi+) = (30.60+/-0.46+/-0.32)%, Gamma(D+-->K(0)K+)/Gamma(D+-->K-pi+pi+) = (6.04+/-0.35+/-0.30)%, and Gamma(D+-->K(0)K+)/Gamma(D+-->K(0)pi+) = (19.96+/-1.19+/-0.96)%.

**0**Bookmarks

**·**

**107**Views

- B. R. Ko, E. Won, I. Adachi, H. Aihara, K. Arinstein, D. M. Asner, T. Aushev, A. M. Bakich, K. Belous, V. Bhardwaj, [......], G. Varner, C. H. Wang, M.-Z. Wang, P. Wang, Y. Watanabe, K. M. Williams, Y. Yamashita, C. C. Zhang, V. Zhilich, A. Zupanc[Show abstract] [Hide abstract]

**ABSTRACT:**We search for CP violation in the decay ${D^{+}}\to K_S^0{K^{+}}$ using a data sample with an integrated luminosity of 977 fb−1 collected with the Belle detector at the KEKB e +e − asymmetric-energy collider. No CP violation has been observed and the CP asymmetry in ${D^{+}}\to K_S^0{K^{+}}$ decay is measured to be (−0.25 ± 0.28 ± 0.14)%, which is the most sensitive measurement to date. After subtracting CP violation due to ${K^0}-{{\overline{K}}^0}$ mixing, the CP asymmetry in ${D^{+}}\to {{\overline{K}}^0}{K^{+}}$ decay is found to be (+0.08 ± 0.28 ± 0.14)%.Journal of High Energy Physics 02/2013; 2013(2). · 5.62 Impact Factor

Page 1

arXiv:hep-ex/0109022v1 17 Sep 2001

Search for CP Violation in the decays D+→ KSπ+and D+→ KSK+

J. M. Link,1M. Reyes,1P. M. Yager,1J. C. Anjos,2I. Bediaga,2C. G¨ obel,2J. Magnin,2A. Massafferri,2

J. M. de Miranda,2I. M. Pepe,2A. C. dos Reis,2S. Carrillo,3E. Casimiro,3A. S´ anchez-Hern´ andez,3C. Uribe,3

F. V´ azquez,3L. Cinquini,4J. P. Cumalat,4B. O’Reilly,4J. E. Ramirez,4E. W. Vaandering,4J. N. Butler,5

H. W. K. Cheung,5I. Gaines,5P. H. Garbincius,5L. A. Garren,5E. Gottschalk,5P. H. Kasper,5A. E. Kreymer,5

R. Kutschke,5S. Bianco,6F. L. Fabbri,6A. Zallo,6C. Cawlfield,7D. Y. Kim,7A. Rahimi,7J. Wiss,7R. Gardner,8

A. Kryemadhi,8Y. S. Chung,9J. S. Kang,9B. R. Ko,9J. W. Kwak,9K. B. Lee,9H. Park,9G. Alimonti,10

M. Boschini,10P. D’Angelo,10M. DiCorato,10P. Dini,10M. Giammarchi,10P. Inzani,10F. Leveraro,10

S. Malvezzi,10D. Menasce,10M. Mezzadri,10L. Milazzo,10L. Moroni,10D. Pedrini,10C. Pontoglio,10F. Prelz,10

M. Rovere,10S. Sala,10T. F. Davenport III,11L. Agostino,12V. Arena,12G. Boca,12G. Bonomi,12G. Gianini,12

G. Liguori,12M. M. Merlo,12D. Pantea,12S. P. Ratti,12C. Riccardi,12I. Segoni,12P. Vitulo,12H. Hernandez,13

A. M. Lopez,13H. Mendez,13L. Mendez,13A. Mirles,13E. Montiel,13D. Olaya,13A. Paris,13J. Quinones,13

C. Rivera,13W. Xiong,13Y. Zhang,13J. R. Wilson,14K. Cho,15T. Handler,15R. Mitchell,15D. Engh,16

M. Hosack,16W. E. Johns,16M. Nehring,16P. D. Sheldon,16K. Stenson,16M. Webster,16and M. Sheaff17

(The FOCUS Collaboration)

1University of California, Davis, CA 95616

2Centro Brasileiro de Pesquisas F´isicas, Rio de Janeiro, RJ, Brasil

3CINVESTAV, 07000 M´ exico City, DF, Mexico

4University of Colorado, Boulder, CO 80309

5Fermi National Accelerator Laboratory, Batavia, IL 60510

6Laboratori Nazionali di Frascati dell’INFN, Frascati, Italy I-00044

7University of Illinois, Urbana-Champaign, IL 61801

8Indiana University, Bloomington, IN 47405

9Korea University, Seoul, Korea 136-701

10INFN and University of Milano, Milano, Italy

11University of North Carolina, Asheville, NC 28804

12Dipartimento di Fisica Nucleare e Teorica and INFN, Pavia, Italy

13University of Puerto Rico, Mayaguez, PR 00681

14University of South Carolina, Columbia, SC 29208

15University of Tennessee, Knoxville, TN 37996

16Vanderbilt University, Nashville, TN 37235

17University of Wisconsin, Madison, WI 53706

(Dated: February 7, 2008)

A high-statistics sample of photo-produced charm from the FOCUS (E831) experiment at Fermi-

lab has been used to search for direct CP violation in the decays D+→ KSπ+and D+→ KSK+.

We have measured the following asymmetry parameters relative to D+→ K−π+π+: ACP(KSπ+) =

(−1.6±1.5±0.9)%, ACP(KSK+) = (+6.9±6.0±1.5)% and ACP(KSK+) = (+7.1±6.1±1.2)% rel-

ative to D+→ KSπ+.The first errors quoted are statistical and the second are systematic.

We have also measured the relative branching ratios and found:

K−π+π+) = (30.60 ± 0.46 ± 0.32)%, Γ(D+→¯ K0K+) / Γ(D+→ K−π+π+) = (6.04 ± 0.35 ±

0.30)% and Γ(D+→¯ K0K+) / Γ(D+→¯ K0π+) = (19.96 ± 1.19 ± 0.96)%.

Γ(D+→ ¯ K0π+) / Γ(D+→

PACS numbers: 11.30.Er 13.20.Fc 14.40.Lb

CP violation occurs when the decay rate of a parti-

cle differs from that of its CP conjugate [1].

Kobayashi-Maskawa ansatz this arises due to the non-

vanishing phase in the Cabibbo-Kobayashi-Maskawama-

trix when the decay amplitude has contributions from

at least two quark diagrams with differing weak phases.

In addition final state interactions (FSI) must provide a

strong phase shift. In the Standard Model direct CP vi-

olation in the charm meson system is predicted to occur

at the level of 10−3or below [2]. The mechanism usu-

ally considered is the interference of the tree and penguin

amplitudes in singly-Cabibbo suppressed (SCS) decays.

In the

In the decay D+→ KSπ+the Cabibbo favored (CF)

and doubly-Cabibbo suppressed (DCS) amplitudes con-

tribute coherently with, perhaps, a different weak phase.1

In addition the isospin content of the DCS amplitude

differs from that of the CF case so we can expect a

non-trivial strong phase shift. Several authors have com-

mented on the effect of K0mixing on the CP asymmetry

for this decay mode and the possibility of using it to

1The charge conjugate state is implied unless stated otherwise.

Page 2

2

search for new physics [3, 4].

Differences in the non-leptonic decay amplitudes of

charmed mesons are almost certainly due to FSI. These

effects tend to be amplified in the charmed system mak-

ing it an ideal laboratory for their study [5]. The isospin

amplitudes and phase shifts in D → KK, D → Kπ and

D → ππ decays can be extracted from measurements of

the branching fractions [6]. For example the magnitude

of the I=3/2 amplitude can be obtained directly from the

D+→¯K0π+partial width [7].

Previous studies of D+→ KSπ+and D+→ KSK+

have concentrated on measuring relative branching ratios

[8, 9]. This paper reports the first measurement of the

CP asymmetry for these decays.

The data were collected during the 1996–1997 fixed

target run at Fermilab. Bremsstrahlung of electrons and

positrons with an endpoint energy of approximately 300

GeV produces a photon beam. These beam photons in-

teract in a segmented beryllium-oxide target and produce

charmed particles. The average photon energy for events

which satisfy our trigger is ≃ 180 GeV. FOCUS uses

an upgraded version of the E687 spectrometer which is

described in detail elsewhere [10]. Charged decay prod-

ucts are momentum analyzed by two oppositely polarized

dipole magnets. Tracking is performed by a system of sil-

icon vertex detectors in the target region and by multi-

wire proportional chambers downstream of the interac-

tion. Particle identification is performed by three thresh-

oldˇCerenkov counters, two electromagnetic calorimeters,

an hadronic calorimeter, and by a system of muon detec-

tors.

The D+→ K−π+π+decay is reconstructed using a

candidate driven vertexing algorithm. A decay vertex

is formed using the reconstructed tracks after which the

momentum vector of the parent D meson is intersected

with other tracks in the event to form a production ver-

tex. The confidence level of the secondary vertex is re-

quired to be greater than 1%. The likelihood for each

charged particle to be an electron, pion, kaon or proton

based on the light yield from each thresholdˇCerenkov

counter is computed [11]. We demand that the Kaon

hypothesis WK, (i.e. −2ln(kaon likelihood)), be favored

over the pion hypothesis Wπ by ∆W = Wπ− WK ≥

1.We also make a pion consistency cut by finding

the alternative minimum hypothesis Wminand requiring

Wmin− Wπ > −2 for both pions. We eliminate con-

tamination due to D∗+→ D0(→ K−π+)π+by asking

that neither Kπ invariant mass combination lies within

25 MeV/c2of the nominal D0mass.

The techniques used for KS reconstruction are de-

scribed elsewhere [12]. Because 90% of KSdecays occur

after the KS has passed through the silicon strip detec-

tor we are unable to employ the same vertexing algorithm

used to reconstruct the D+→ K−π+π+decay. Instead

we use the momentum information from the KS decay

and the silicon track of the charged daughter to form a

FIG. 1: Invariant mass plots for D+→ KSπ+and

D−→ KSπ−

candidate D vector. This vector is intersected with candi-

date production vertices which are formed from two other

silicon tracks. As a final check we force the D vector to

originate at our production vertex and calculate the con-

fidence level that it verticizes with the charged daughter.

This confidence level must be greater than 2%. We re-

quire that the momentum of the charged daughter be

greater than 10 GeV/c, that the confidence level for it to

be a muon be less than 1%, and that it traverse the entire

length of the spectrometer. For the decay D+→ KSπ+

we demand that Wmin−Wπ> −6 and Wπ−WK< 0, for

D+→ KSK+we ask that the kaon hypothesis be favored

over both the proton and pion hypotheses by requiring

Wp− WK> 0 and Wπ− WK> 3. We remove electron

contamination by ensuring that the charged D daughter

links to only one silicon microstrip track. Electron pairs

usually have a very small opening angle in the silicon and

chamber tracks tend to link to both tracks. Checks for

electron contamination of the KSsample using the elec-

tromagnetic calorimeters showed no significant effect. We

use only KS candidates which have a normalized mass2

within three standard deviations of the nominal value.

Additionally, to reduce backgrounds in the D+→ KSK+

mode, we do not use the category of KSdecays which oc-

cur downstream of the silicon where both KSdaughters

lie outside the acceptance of the downstream magnet. We

make the same cut on the D+→ KSπ+normalization

2The normalized mass is the difference between the measured and

the nominal mass divided by the error on the measured mass.

Page 3

3

FIG. 2: Invariant mass plot for D+→ KSK+and

D−→ KSK−. The shaded area is the smoothed background

shape from D+

s → K∗+¯ K0and D+

s →¯ K∗0K+.

signal. When the KS decays in the silicon detector we

demand that all three tracks be inconsistent with origi-

nating at the same vertex. This eliminates backgrounds

from decays such as D+→ π−π+π+.

For all modes we require that the production vertex

have a confidence level greater than 1%, that the maxi-

mum confidence level for a candidate-D daughter track

to form a vertex with tracks from the primary vertex

be less than 20%, that the significance of separation of

the production and decay vertices be greater than 7.5

and that both vertices lie upstream of the first trigger

counter. The momentum of the D must be greater than

40 GeV/c. In Figures 1, 2 and 3 we show the invari-

ant mass distributions for the decays KSπ+, KSK+and

K−π+π+respectively.

We construct the CP asymmetry, ACP as the difference

in the yields, (corrected for efficiency and acceptance),

of the decay in question divided by the sum. We must

also account for differences in production between the

D+and D−. To do this we ratio the corrected yields to

those of a Cabibbo favored decay which is assumed to be

CP conserving. We measure:

ACP=η(D+) − η(D−)

η(D+) + η(D−)

where for example,

η(D+) =

N(D+→ KSπ+)

N(D+→ K−π+π+)

is the ratio of the corrected yields for each decay which

is equivalent to the relative branching ratio.

FIG. 3: Invariant mass plot for D+→ K−π+π+and

D−→ K+π−π−. The shaded area is the third-degree

polynomial background in the fit region.

To account for non-Gaussian tails in the D+

K−π+π+signals we find it necessary to fit these distribu-

tions using two Gaussians and a third-degree polynomial.

The KSπ+distribution is fit using a Gaussian and a

linear polynomial. The non-linear background shape be-

low 1.75 GeV/c2in the KSπ+plot is primarily due to

D+→ KSl+νland is not included in the fit.

We fit the KSK+signal using a combination of a

Gaussian, linear polynomial, and a background shape

derived from Monte Carlo. This shape is a smoothed

fit to D+

due to a missing π0, are responsible for the background

shape below the D+peak. Because of the difficulty in

fitting the region between the D+and D+

KSK+distribution we only fit up to 1.935 GeV/c2. To

minimize systematic errors we change the KS selection

cuts on the D+→ KSπ+normalization signal to match

those used for the D+→ KSK+mode. The yield for the

decay D+→ KSπ+changes to 4487±96 events and for

D−→ KSπ−becomes 4770±96 events. We can now cal-

culate the relative branching ratios and CP asymmetries.

The results are shown in Tables I and II.

We studied systematic effects due to uncertainties in

our Monte Carlo production model, reconstruction al-

gorithm, and variations in our selection cuts. For the

D+→ KSπ+measurements we split the sample into

eight statistically independent subsamples based on D+

momentum, loose and tight normalized KS mass cuts,

and the time period in which the data were collected.

The momentum dependence of the result arises mainly

due to uncertainties in the parameters used to generate

→

s→ K∗+¯K0and D+

s→¯K∗0K+decays which,

speak in the

Page 4

4

TABLE I: Relative branching ratio results. The first error

is statistical and the second is systematic. We account for

the decay chain ¯ K0→ KS → π+π−by multiplying our

KS numbers by a factor of 2.91 assuming that

¯ K0π+) = 2 × Γ(D+→ KSπ+) ; we then quote these results

in terms of¯ K0.

MeasurementResult

Γ(D+→¯

K0π+)

Γ(D+→K−π+π+)(30.60 ± 0.46 ± 0.32)%

Γ(D+→¯

K0K+)

Γ(D+→K−π+π+)

(6.04 ± 0.35 ± 0.30)%

Γ(D+→¯

K0K+)

Γ(D+→¯

Γ(D+→

PDG Average [13]

(32.0 ± 4.0)%

(7.7 ± 2.2)%a

K0π+)(19.96 ± 1.19 ± 0.96)%(26.3 ± 3.5)%

aThis is the measurement of reference 6 with statistical and sys-

tematic errors added in quadrature.

TABLE II: CP asymmetry measurements. The first error is

statistical and the second is systematic.

Measurement

ACP(KSπ+) w.r.t. D+→ K−π+π+

ACP(KSK+) w.r.t. D+→ K−π+π+(+6.9 ± 6.0 ± 1.5)%

ACP(KSK+) w.r.t. D+→ KSπ+

Result

(−1.6 ± 1.5 ± 0.9)%

(+7.1 ± 6.1 ± 1.2)%

our Monte Carlo. The D+→ KSπ+topology and recon-

struction algorithm is substantially different from that of

the D+→ K−π+π+and the two modes differ in how

well the Monte Carlo matches to the data. For example

there is a slight difference in how well the generated and

accepted momentum distributions agree in each case. We

use a technique modeled after the S-factor method used

by the Particle Data Group [13] to evaluate the system-

atic error. A scaled variance is calculated using the eight

independent subsamples. The split sample systematic is

defined as the difference between the scaled variance and

the statistical variance when the former exceeds the lat-

ter. Due to the smaller statistics in the D+→ KSK+

decay mode we can only form four independent subsam-

ples. These are based on the run period in which the

data were collected and on the normalized KSmass.

We evaluate systematic uncertainties due to the fitting

procedure by calculating our results for various fit con-

ditions, such as rebinning the histograms, changing the

background shapes and in the case of D+→ KSK+also

fitting the Dspeak. Since these different results are all a

priori likely we use the resulting sample variance as a sys-

tematic. The total systematic is calculated by adding the

fit-variant systematic and the split-sample systematic in

quadrature. For the D+→ KSπ+measurements the sys-

tematic has contributions from both the split-sample and

fit-variant analyses. For the Γ(D+→¯K0π+) / Γ(D+→

K−π+π+) measurement the contribution from the split-

sample is 0.301% and from the fit-variant 0.098%. For the

ACP(KSπ+) measurement the split-sample contribution

is 0.92% and that of the fit-variant is 0.13%. We find no

systematic contribution to the D+→ KSK+measure-

ments from the split-sample technique, and therefore the

fit-variant contributions are identical to the total system-

atic error and are as shown in Tables I and II. Due to the

lower statistics we did not split the D+→ KSK+sample

by momentum. Instead we treat the weighted average of

two samples split by momentum as a fit variant.

To conclude, we have searched for evidence of direct

CP violation in the decays D+→ KSπ+and D+→

KSK+and measured their branching ratios relative to

each other and to D+→ K−π+π+. Our relative branch-

ing ratios are a considerable improvement over previous

measurements. The CP asymmetries have not been pre-

viously measured for these modes and are consistent with

zero.

We wish to acknowledge the assistance of the staffs

of Fermi National Accelerator Laboratory, the INFN of

Italy and the physics departments of the collaborating

institutions. This research was supported in part by the

National Science Foundation, the U.S. Department of En-

ergy, the Italian Istituto Nazionale di Fisica Nucleare and

Minsitero dell’Universit` a e della Ricerca Scientifica e Tec-

nologica, the Brazilian Conselho Nacional de Desenvolvi-

mento Cient´ ıfico e Tecnol´ ogico, CONACyT-M´ exico, the

Korean Ministry of Education and the Korean Science

and Engineering Foundation.

[1] I. Bigi and A. Sanda, CP Violation (Cambridge Uni-

versity Press, The Edinburgh Building, Cambridge CB2

2RU, UK, 2000).

[2] F. Buccella, M. Lusignoli, G. Miele, A. Pugliese, and

P. Santorelli,Phys. Rev. D51, 3478 (1995),

ph/9411286.

[3] I. I. Bigi and H. Yamamoto, Phys. Lett. B349, 363

(1995), hep-ph/9502238.

[4] H. J. Lipkin and Z.-Z. Xing, Phys. Lett. B450, 405

(1999), hep-ph/9901329.

[5] J. L. Rosner, Phys. Rev. D60, 114026 (1999), hep-

ph/9905366.

[6] M. Bishai et al. (CLEO), Phys. Rev. Lett. 78, 3261

(1997), hep-ex/9701008.

[7] M. Bauer, B. Stech, and M. Wirbel, Z. Phys. C34, 103

(1987).

[8] J. C. Anjos et al., Phys. Rev. D41, 2705 (1990).

[9] P. L. Frabetti et al. (E687), Phys. Lett. B346, 199

(1995).

[10] P. L. Frabetti et al. (E-687), Nucl. Instrum. Meth. A320,

519 (1992).

[11] J. M. Link et al. (FOCUS), accepted for publication in

NIM-A, FERMILAB-Pub-01/243-E, hep-ex/0108011.

[12] J. M. Link et al. (FOCUS), submitted to NIM-A,

FERMILAB-Pub-01/244-E.

[13] D. E. Groom et al. (Particle Data Group), Eur. Phys. J.

C15, 1 (2000).

hep-

#### View other sources

#### Hide other sources

- Available from Marco A. Reyes · Aug 21, 2014
- Available from ArXiv